I thought that that was how Usenet worked: each news server would merge content from other servers into the same newsgroup, then everyone would see the same alt.urban.legends or whatever.

By people creating their own groups of subs I mean a group saved on their private app or profile, not some other huge functionality to the system creating lists that is shared publicly, that would require a central authority an major changes. Heck, an app like jerboa could probably do it itself. The people that want to migrate every iteration of a subject into one don't get the fediverse. But being able to add like communities to a list yourself gets rid of the fractured communities downside of the fediverse. If you stumble on another community on the subject you add it to that catagory/group. It would also increase the users investment and loyalty to their account.